1. Field of the Invention
The present invention relates to a token passing type slotted loop network system wherein a plurality of node stations are connected by transmission lines to form a data transmission ring or loop, each station sending out its own data signal in a slot or slots on the loop after capturing the token, and in particular, to frame stripping, access control and priority control in such a token passing type slotted loop network system.
2. Description of the Prior Art
In such a slotted loop network system, a particular one of the node stations transmits its own data signal by use of one or more of the plurality of slots. However, the particular node station is responsible for stripping from the loop the slot which is used for transmitting the own data signal. The stripping is performed by use of address data in the own data signal or by node address data attached into the used slot. However, it takes a long time to detect the address data so that stripping is delayed.
As the data signal to be transmitted, are two types, one being a simple packet signal having a source address and a destination, the other being an ATM (Asynchronous Transfer Mode) type having a different address data signal which is called a VCI (Virtual Channel Identifier) which is different from those source and destination address type. In the loop network system handling those different type data signals, the stripping is further complex.
In a conventional token passing ring (IEEE802.5), a token frame is circulated through the loop. When a particular one of node stations acquires the token frame, the particular node station has a transmission right to transmit the own data signal together with the token to the loop. However, when the transmission of the own data signal is ended, the token cannot be freed until the final data frame returns to the particular node station. Therefore, the process for acquiring the token frame by another node station is comparatively complex and takes a long time.
In order to enable a high speed access control, a method is known in the prior art wherein the particular node station frees the token just after completion of the transmission of the own data signal. The FDDI (Fiber Distributed Data Interface) uses such a method. However, it is impossible in the method to perform the priority control of transmission requests originated from several node stations.